Why Some Race Cars Kept Backflipping In the Late 1990s

If you watched the 1999 Le Mans 24 Hours, you undoubtedly remember Peter Dumbreck's spectacular crash on the way to Indianapolis corner, where his Mercedes CLR took flight, seemingly for no discernible reason, flipping through the air multiple times before landing in the woods. It happened to his teammate, Mark Webber, at the same spot during qualifying and just before Mulsanne Corner in practice, too.

The CLR's flights were foreshadowed by a similar crash at the inaugural Petit Le Mans, the prior year. Yannick Dalmas's Porsche 911 GT1 became airborne after cresting a hill, backflipping once before landing on its rear. The same thing happened to Bill Auberlin at the same race two years later, though he was driving a BMW V12 LMR.

The aftermath of Webber’s crash near Mulsanne Corner at Le Mans practice in 1999.

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So why did all these cars crash in almost the same exact way around the same time? As Mike Fernie explains in this Drivetribe video, it has to do with the aerodynamic design of top-level sports racing cars of the era.

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Typically, a race car uses a negative pitch angle, where the front of the car sits slightly lower than the rear, increasing frontal area. This is to help generate downforce, and thus, increase cornering speeds. The 911 GT1, CLR and V12 LMR were all designed primarily for Le Mans, though, a circuit that's defined by its long Mulsanne straight. Even after two chicanes were added in 1990 to break up the 3.6-mile straight, top speed remains of critical importance at this track.

Porsche, Mercedes and BMW all adopted more neutral pitch angles, reducing frontal area in the name of higher speeds on the straights. The problem is when cresting a hill, pitch angle actually becomes positive, which can actually create a lifting force under the front of the car. As Fernie explains, once the pitch angle of the Mercedes reached 2.4-degrees, all the downforce at the rear was gone, and the car went airborne.

After Dumbreck's crash during the 1999 race, Mercedes pulled out its two remaining cars, and to this day, the automaker hasn't returned to Le Mans. To prevent this sort of thing from ever happening again, the FIA mandated that prototypes needed vents in their front fenders to reduce air pressure buildup at the front axle. Front overhangs were also decreased, as the CLR's especially long overhang exacerbated the negative side effects of increasing pitch angle. The hill at Indianapolis was smoothed over, too.

Front fender vents on an Audi R8 LMP1.

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Miraculously, no one was badly injured in any of these crashes, and none similar have happened since. It's a weird footnote in a fascinating era of sports car racing. Watch Fernie's video below, and for more, check out the analysis at Mulsanne's Corner.

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